In the manufacture of electronic devices it is standard practice to mount or form the various electronic components which comprise the circuits of the devices on a substrate. Various materials have been suggested for use as the substrate, such as organic plastic laminates, metal, porcelain coated steel, and ceramic wafers.
Relatively large circuits such as those employed in radios, televisions, computers and the like are generally produced on organic plastic substrates. The substrates or, as they are more commonly referred to, circuit boards, are typically comprised of a reinforced thermal setting resin. The most widely used type of organic plastic circuit board is comprised of paper reinforced phenolic resin. Another widely used type of organic circuit board is glass fabric reinforced epoxy resin laminate.
The organic plastic circuit boards have many advantages. They are relatively inexpensive and can be manufactured in almost any desired size with very flat smooth surfaces. They also have reasonably good physical strength.
The organic plastic circuit boards also have a number of inherent disadvantages which limit the use of this class of circuit boards. They cannot be exposed to high temperatures, that is temperatures in excess of about 400.degree. C. which limits their use to low temperature manufacturing processes. Required conductive metal circuitry and the like must be formed on the surface of the circuit board using low temperature processes such as metal etching or electrical or chemical metal deposition. The electronic components such as capacitors and, more particuarly, resistors and the like, must be manufactured as discrete components in separate manufacturing operations and then individually mounted on the organic plastic circuit boards.
The high temperature limitation of the organic plastic circuit boards has become a serious manufacturing problem. New methods have been developed to form electronic components directly on the surface of suitable substrates. These so-called process induced components (PIC) offer certain highly desirable advantages such as being relatively low in cost, simple to form and assemble into circuits, and generally having greater overall improved reliability and electrical accuracy. The materials which are used to form the process induced components are usually prepared in the form of inks which are comprised of metal and glass powder. The inks are printed on the substrate in the desired pattern and the substrate is then fired at elevated temperatures to fuse the materials in the ink and form the desired electronic component. The firing temperatures which are required are generally in the range of 600.degree. to 900.degree. C. or even higher. This is considerably in excess of the upper temperature limits to which the organic plastic circuit boards can be exposed without degradation.
It has been suggested to use various types of ceramic materials as the circuit board, especially where the board will be subjected to high temperatures. A ceramic material which has been employed for this purpose is aluminum oxide wafers. The ceramic materials have excellent high temperature resistance and can be fired and refired many times at temperatures of 600.degree. to 900.degree. C. and even higher without any adverse effects. The ceramics would be ideal substrates for electronic circuits except for the fact that ceramics have certain serious inherent disadvantages, such as being relatively expensive to manufacture and impractical to manufacture in relatively large sizes because of the fragility of the ceramics. This physical size limitation is a serious problem in that a plurality of separate ceramic boards are required where only one organic plastic board would be required. The ceramics also cannot readily be machined or punched to provide mounting aperatures and the like required for mounting discrete electronic components. The fragile nature of the ceramic substrates also necessitates the use of mounting fixtures and protective shields to prevent damage to the substrate in use.
Suggestions have also been made in the prior art to use porcelain coated steel as a circuit board. Porcelain coated steel would appear to have a combination of the desirable properties of both the organic plastic circuit boards and the ceramic circuit boards. Porcelain coated steel circuit boards can be made in large sizes similar to the organic plastic boards. Prior to porcelainization, the steel cores of the porcelain coated boards can be easily shaped and apertures can be made in steel boards. The porcelain boards are not subject to thermal degradation at low temperatures, for example 400.degree. C., as are the organic plastic circuit boards. In this respect they are similar to the ceramic circuit boards. The porcelain coated circuit boards are, however, much stronger than the ceramic boards and can be employed in relatively rugged applications.
Porcelain coated metal boards were suggested for use as circuit boards at least as early as the 1930's. However, the porcelain coated boards heretofore known have not proven to be satisfactory. This is especially true with regard to porcelain coated boards which are employed for substrates for process induced components.
One of the principal problems encountered with the prior art porcelain coated steel boards is that when the porcelain is fired it does not fuse into a layer of uniform thickness. Excessive porcelain builds up on the edges of the steel cores in the form of raised lips or ridges. In addition, depending upon the type and conditions used for firing, the porcelain forms as either a meniscus about apertures in the board or forms very thin coatings over edges of the holes. This unevenness in the thickness of the coatings on the surface of the prior art porcelain boards makes it difficult if not impossible to accurately print circuits on the surface of the boards.
A further problem which is encountered is that on refiring at even relatively low temperatures of, for example, 500.degree. to 600.degree. C., the porcelain of the prior art resoftens and reflows. This situation becomes more of a problem if the porcelain is subjected to repeated refiring in that the porcelain continues to reflow on refiring. The movement on reflow of the porcelain coating distorts or even destroys the printed electronic components on the surface of the board.
A still further problem encountered with the porcelain coated metal circuit boards of the prior art is that upon reheating to temperatures even slightly above the softening point of the porcelain, there is often an evolution of gases from the metal core of the substrate. These gases then form bubbles in the porcelain coating which cause shorts between the metal core and components on the surface.
Poor adhesion of the porcelain of the prior art to the metal cores is likewise a very serious problem, especially after repeated high temperature firings. This is believed to be due in part to the substantial differences in the coefficients of thermal expansion of the metal cores and the porcelains of the prior art.
Another problem which is encountered with the porcelain boards of the prior art and which has been a major problem, is known as brown plague. This condition appears to occur with the prior art porcelain boards when an inadvertent electrical contact is made between a conductor on the surface of the board and the metal core of the board. An electrical degradation of the dielectric properties of the porcelain occurs which over a period of time leads to functional failure of the board.
These problems have led to only limited acceptance of porcelain metal boards. It would be highly advantageous if a porcelain coated steel board could be provided which would have the advantages of the organic circuit boards and also the ceramic circuit boards without having the disadvantages noted above of the prior art porcelain coated metal circuit boards.